> Concept: in other statements (add and delete) in the query,
Categories:
(1) to a location:
(1). Select back:
Support only a scalar subquery (only one line of a result set column),
2. The from behind:
Support table subquery (result sets multi-line column),
(3). Where or having behind:
Scalar subquery, example queries, muck query,
(4). Behind the exists (correlated sub-query)
A table subquery,
(2) according to the result set ranks number:
(1). A scalar subquery result set a row (column),
Diction. Example query result set (only more than one column),
(3). Muck query (result sets have more than one column),
(4). A table subquery (result sets general multi-line column),